Replace one of those bolts fir the Bumperette stiffener with a nice shiny Bolt and lockwasher and nut after you run a tap through the captive Nut, I think it has a captive nut. Add a closed loop lug on the end of the ground wire and place over that shiny bolt you fed up from underneath the car, Add the other two ground lugs that have been soldered, heat shrink tape to neaten up the connection and when all 3 wires are on each side add a lockwasher and another nut on top. You will have no more issued with a bad ground back there. Again there are no ground wires in the rear harness. Those bulb all ground to the body. Yes I could have run one continuous ground wire but 3 separate wires made trouble shooting and isolation simpler. Dont foget the ground to the Plate Lamp.
Now one trick I added was a 1157 dual filament lower beehive lamp in the back, same base as the front turn signals. I ran a jumper wire from the brake lamps down to the low beam side of the new beehive lamp. Step on the brakes and it also lights up the low intensity side of the beehive lamp. Turn Signal stays wired to the high intensity side. All of my rear bulbs are now LED, I added the 72 LED Display for main Brake Lamps and 1157 Red / Red dual filament LED's in the Beehive lamps. UP front I used a White / Amber LED for the front Turn signals. PArking lights White / Turn Signals Amber. Works slick.